1. Field of the Invention
The present invention relates generally to producing dispersions of finely divided particles within a liquid carrier and pertains, more specifically, to apparatus and method for the production of particle dispersions in which particle size is within a range measured in nanometers.
The technology wherein dispersions are produced by utilizing a field of media in which solids are ground within liquids has demonstrated that the quality of such dispersions can be enhanced by significantly reducing the size of the particles present in the finished dispersion. Immersion mills, such as those of the type described in U.S. Pat. No. 5,184,783, the disclosure of which is incorporated herein by reference thereto, have been employed to process feedstock through a bed of media to create dispersions of consistent high quality. However, efforts to increase even further the quality of such dispersions by reducing particle size down to a range measured in nanometers, that is, to a size less than one micron in diametric dimensions, have met with difficulties in separating the very small media required in the media field from the feedstock during the conduct of the grinding process. Conventional apparatus and method which utilize screening devices or gap separation devices for separating the media from the feedstock and confining the media to the bed of media tend to clog readily, thereby reducing flow and providing very low levels of throughput.
2. Description of Related Art
The problem of reduced flow and concomitant low levels of throughput has been addressed successfully by the use of immersion mills provided with porous containment walls, as described in U.S. Pat. Nos. 7,828,234 and 7,883,036, the disclosures of which are incorporated herein by reference thereto. However, it has been found that upon reducing the size of the media employed in such immersion mills into the range of sizes required to attain a dispersion wherein particle size is in the nanometer range, even though the containment wall is effective in precluding escape from the media field of the very small media necessary for the conduct of such a process, the very small media have found an alternate path of escape, namely, through manufacturing tolerances existing at the lower bearing that provides an internal support for the bottom impeller employed in such immersion mills, as illustrated in the aforesaid U.S. Pat. No. 5,184,783, wherein a bottom impeller (150) is supported by a bearing (86). Thus, where the media size is reduced to below about 0.3 mm, media can find a path out of the media field, through the clearance provided by the tolerances present between the bearing and the rotating shaft that extends through the bearing. While a hub construction as described in U.S. Pat. No. 7,559,493, the disclosure of which is incorporated herein by reference thereto, has been found effective in deflecting media away from such an escape path where the media falls within a size range of no less than about 0.3 mm, the construction described in U.S. Pat. No. 7,559,493 has been found unable to preclude the migration and escape of media having diametric dimensions less than about 0.3 mm, particularly when the media field is at rest and the hub is not rotating.